The present invention relates generally to seals between circumferentially registering endfaces of packing seal segments in a steam turbine and particularly relates to spline seals between endfaces of packing seal segments circumferentially disposed end-to-end in a stationary packing casing of the steam turbine and methods of retrofitting the spline seals.
In a steam turbine, there are circumferentially extending packing seal segments disposed in the packing or stationary casing at opposite ends of the turbine and in sealing relation with the rotor. In certain steam turbines, the packing seal segments are provided intermediate pressure opposite ends of the rotor between high and intermediate turbine sections. These packing seal segments minimize or preclude steam leakage from the rotor ends or through leakage paths between the high and intermediate pressure sections of the steam turbine within the stationary casing. It will be appreciated that the sealing segments extend circumferentially and have endfaces in circumferential registration with one another. The endfaces of the sealing segments are not sealed to one another and the resulting steam leakage flows detrimentally impact overall machine performance. Further, it is not possible to maintain zero clearance between the segment endfaces for a number of reasons, including manufacturing tolerances which dictate a gap to accommodate the tolerance range. Additionally, machine thermal transients may cause the segments to bind in assembly, even if designed line to line. Further, most seal segments are movable radially inwardly and outwardly. This requires a gap between the segment endfaces, at least in radially outermost positions of the segments. Further, if the casing surrounding the rotor goes out of round, the gaps may become large in one location versus another location. Still further, if the casing grows thermally more than the packing segments, then the end gaps may increase in dimension relative to the cold condition of the turbine. Steam leakage flows between endfaces of packing seal segments have therefore become a significant problem, and, accordingly, there is a need for minimizing leakage paths for steam past the endfaces of circumferential packing seal segments in a steam turbine.
In accordance with a preferred embodiment of the present invention, there are provided circumferentially extending packing ring segments disposed in a packing casing surrounding the rotor. The segments have spline seals between the circumferentially registering endfaces for minimizing leakage flow past the endfaces. The registering endfaces of adjacent segments are provided with grooves for receiving portions of the spline seal. The spline seal may comprise a flat sheet metal plate extending between circumferentially registering grooves arranged either in a generally axial direction to preclude radial leakage flow or at an angle to or normal to the turbine axis to preclude axial leakage flow. The spline seals per se may be wrapped with metallic cloth. The spline seals may also have enlargements at opposite ends for seating in the bases of the registering grooves. Thus, the central portion of the spline seal bridging the gap between the segments is spaced from the sides of the grooves, and enables relative movement of the segments in a direction normal to the spline seal without binding or severing of the spline seal.
The sealing ring segments are employed at opposite ends of the steam turbine at axially spaced locations from the turbine section. Thus, the spline seals between endfaces of the sealing segments preclude axial leakage flow outwardly of the turbine section. The spline seals may also be utilized with seal segments disposed between high and intermediate pressure turbine sections to minimize leakage from the high pressure section to the intermediate pressure section.
A particular advantage of the present invention resides in the ability to retrofit the spline seals to existing steam turbines as a means of improving overall machine performance. To accomplish this, and during a normal outage for maintenance, the upper outer casing of the turbine is removed and the existing seal segments are rolled from the turbine casing. Slots are machined in the endfaces to receive the spline seals. The segments are then rolled back into the upper and lower outer casings with the spline seals inserted between opposing endfaces, thereby reducing steam leakage paths in existing steam turbines after the retrofit.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor having a steam turbine section, a stationary casing surrounding the rotor including the steam turbine section, the casing carrying a plurality of circumferentially extending packing seal segments about the casing at a seal location axially spaced from the turbine section for sealing between the casing and the rotor, each of the segments having endfaces respectively in circumferential registry with opposed endfaces of circumferentially adjacent segments, the endfaces including slots opening circumferentially and in general circumferential registration with one another and a spline seal extending between each of the opposed endfaces of circumferentially adjacent segments and in the slots for minimizing or precluding steam leakage past the registering endfaces.
In a further preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor having a steam turbine section, a stationary casing surrounding the rotor including the steam turbine section, the casing having a circumferentially extending dovetail-shaped groove carrying a plurality of circumferentially extending packing seal segments about the casing in the groove, the segments carrying labyrinth seal teeth for sealing about the rotor, each of the segments having endfaces respectively in circumferential registry with opposed endfaces of circumferentially adjacent segments, the endfaces including slots opening circumferentially and generally in circumferential registration with one another and a spline seal extending between each of the opposed endfaces of circumferentially adjacent segments and in the slots for minimizing or precluding steam leakage past the registering endfaces.
In a further preferred embodiment according to the present invention, there is provided in a turbine having a rotor, a stationary casing surrounding the rotor and a plurality of circumferentially extending packing ring segments in circumferentially extending grooves about the casing for sealing between the casing and the rotor, a method of retrofitting the packing ring segments to provide seals between the opposed endfaces of adjacent packing ring segments comprising the steps of removing the packing ring segments from the turbine, forming at least one slot in each endface of the removed packing ring segments, disposing a spline seal in slots of opposed endfaces of the packing ring segments and inserting the packing ring segments into the grooves of the casing whereby the spline seals extend between adjacent segments for minimizing or precluding steam leakage flows between the adjacent segments.